Currently, in the field of electronic devices such as flat panel displays and electronic papers, as substrates, glass substrates are mainly used. However, a glass substrate is heavy and fragile and thus is not necessarily an ideal substrate. Therefore, studies have been extensively conducted to realize a flexible device in which a glass substrate is replaced with a substrate made of a polymer material. However, many of the technologies for producing such flexible devices require new production techniques and equipment. Therefore, flexible devices using polymer materials have not yet mass-produced.
On the other hand, recently, as a shortcut for efficiently mass-producing flexible devices, it has been proposed to produce a flexible device using an ordinary process for a glass substrate by using a laminate that is obtained by forming a polyimide resin layer on a glass substrate (Non-Patent Document 1). In the process using the laminate, the flexible device is obtained by separating the polyimide resin layer from the glass substrate at a final stage.
In such a process, the laminate is required to have smoothness and low warpage for good handling. That is, the polyimide resin layer of the laminate needs to have a linear expansion coefficient that is about the same as that of glass. As a material of glass substrate, soda-lime glass and alkali-free glass are generally used. The soda-lime glass has a linear expansion coefficient of about 8-9 ppm/° C., and the alkali-free glass has a linear expansion coefficient of about 3-5 ppm/° C. Further, when an amorphous silicon thin film transistor is manufactured, temperature of the process reaches a maximum of 300-350° C. The linear expansion coefficient of a common polyimide is larger than that of glass. Therefore, materials suitable for such a process are limited in nature. For example, Patent Document 1 discloses a method in which a laminate is obtained by casting and thermally imidizing a polyimide precursor solution obtained from 3,3′,4,4′-biphenyl-tetracarboxylic acid dianhydride and p-phenylenediamine or 4,4″-diamino-para-terphenyl on an inorganic substrate. On the other hand, when a polyimide precursor having a specific structure is formed into a film on an inorganic substrate and is further thermally imidized by raising temperature at a certain speed or faster, the polyimide film may peel off from the substrate. Therefore, for a purpose of improving adhesion between polyimide and an inorganic substrate, the inorganic substrate is subjected to a surface treatment (Non-Patent Document 2), or a silane coupling agent having an amino group or an acid anhydride group is added to a polyimide precursor solution (Patent Documents 2 and 3).